1. Field of the Invention
The present invention relates generally to an ignition coil of an internal combustion engine which generates a high voltage to be applied to a spark plug, and more particularly to the ignition coil in which a center core is formed of compressed magnetic powder compressed and molded by divided dies.
2. Description of Related Art
An ignition coil has a primary coil and a secondary coil performing mutual induction with each other, and the primary coil has a center core. To enhance the effect of the mutual induction, the center core are disposed inside both the primary coil and the secondary coil. As the center core, a laminated core having the lamination of silicon steel sheets and a compressed powder core, obtained by compressing and molding particles of magnetic powder covered with an insulator, are well known.
Because the surface of the compressed powder core is smooth or glassy as compared with the surface of the laminated core, the compressed powder core has been used as a center core. In recently, to minimize the ignition coil, a winding is directly wound on the compressed powder core without using any winding frame. Further, the compressed powder core can be easily molded in a complicated shape, as compared with the laminated core. Therefore, a winding body portion of the compressed powder core is formed in a columnar shape with a step, and the winding is wounded on the winding body portion. In this case, the number of turns in the winding can be increased without enlarging the compressed powder core.
Published Japanese Patent First Publication No. 2007-324274 discloses a method of producing a compressed powder core of low iron loss. In this method, a mechanical shock is given to raw material of magnetic powder produced according to the water atomizing method to form each of particles of the magnetic powder in a spherical shape, the magnetic powder is annealed to remove distortion from the magnetic powder, the magnetic powder is covered with an insulator made of a component of heat resisting organic resin such as silicon resin to insulate the particles of the magnetic powder from one another, and warm compression molding is performed for the magnetic powder to form a compressed powder core of low iron loss. This compressed powder core used as the center core of an ignition coil is composed of a winding body portion, formed in a columnar shape with a step, and a collar portion. The body portion has a larger diameter portion and a smaller diameter portion. The collar portion extends from the end of the smaller diameter portion in directions, being perpendicular to the center axis of the body portion, substantially in a rectangular shape, and is formed in a rectangular parallelepiped shape. A winding is wound on the body portion. The collar portion fixedly catches a winding start portion and a winding end portion of the winding. The collar portion is inserted into a connector socket, so that this socket receiving the collar portion fixedly holds the center core.
Because the outer shape of the compressed powder core used for the ignition coil has a step, when this core is formed by using an integrally-formed molding die, it is difficult to release the core from the die. Therefore, a plurality of divided dies are prepared by dividing a molding die having a step shape, and the compressed powder core is formed by using these divided dies. However, when magnetic powder is compressed and molded by using these divided dies, burrs are inevitably formed on the outer circumferential surface of the compressed powder core along dividing lines between the divided dies. More specifically, to prevent the dies from being broken, edges of each divided die placed on contact faces to be in contact with faces of the other divided die are cut out in advance to form chamfered planes on the contact faces of the divided die. These chamfered planes form openings along the dividing lines of the divided dies when the divided dies are brought into contact with one another. Further, other small openings are formed along the dividing lines of the divided dies due to wear caused by the use of the divided dies. Therefore, when magnetic powder is packed into the dies, the powder goes into these openings and is compressed to form burrs. These burrs extend along the dividing lines of the divided dies, on the outer circumferential surface of the compressed powder core.
When a winding is directly wound on the compressed powder core with these burrs, the insulator covering the coil is cut by the burrs, and the primary coil sometimes causes a short circuit. To prevent the primary coil from causing a short circuit, it is required to remove burrs formed on the surface of the compressed powder core by grinding or the like. Burrs formed on the outer circumferential surface of the body portion, of which the diameter is gradually changed, can be easily removed by using a well-known method such as buffing or blasting.
However, it is difficult to perfectly remove burrs, formed on a root of the winding body portion extending from the collar portion to be perpendicular to the collar portion, by a simple method. Therefore, burrs formed in the root sometimes remain slightly.
Further, the compressed powder core has a low tenacity or toughness. Therefore, when the compressed powder core is integrally formed with a connector socket by an insert molding, the collar portion is sometimes broken. To prevent this brakeage of the collar portion, it is required to insert the collar portion into a connector socket formed in a box shape by a so-called outsert molding and to form a unit of the socket and the compressed powder core fixed into the socket. In this molding, a portion of a side wall of the Socket forming the box is cut to form an opening. The root of the winding body portion passes through the opening during the insertion of the collar portion to be placed in the opening.
However, in this unit of the socket and the compressed powder core inserted into the socket by the outsert molding, burrs remaining on the root of the body portion are undesirably protruded from the opening of the socket. Therefore, when a winding is wound on the body portion of the compressed powder core by a predetermined number of turns in a multi-layer, one turn of winding in the second layer is pushed toward an opening between a first turn of winding nearest to the collar portion and a second turn of winding adjacent to the first turn in the first layer. Because a winding portion, not in contact with the side wall of the socket but facing the opening, in the first turn of winding is not supported by the side wall, this winding portion is bent toward the collar portion. In this case, the burrs protruded from the opening of the socket sometimes damage the insulator of the first turn of winding so as to cause a short circuit in the primary coil.